Methods for synchronizing in a wide band code division multiple access communication system

ABSTRACT

The invention discloses a method that implements synchronization for a WCDMA system. The method introduces a globe synchronization time to the WCDMA system as a reference time for signal transmitting of all NodeBs; when searching adjacent cells or making handover, each UE takes this reference time as a reference time; in this way, the UE design complexity is reduced, the UE available time is increased, the handover signaling is simplified and the handover success and reliability are increased too. Therefore, the method increases system capacity and makes service quality better.

FIELD OF THE TECHNOLOGY

The present invention generally relates to synchronization technique ofa Wideband Code Division Multiple Access (WCDMA) system, especially toimplementation method that takes the Global Synchronization time as thestandard reference time for transmission of all base stations in a WCDMAsystem.

BACKGROUND OF THE INVENTION

Since 1980s the CDMA technique has been used in a digital mobilecommunication field and becomes a vital mobile communication mode withthe high spectrum availability and powerful anti-interferencecapability. At present, people pay more attention on the thirdgeneration mobile communication system and have done a lot of work on itsuch as Future Public Land Mobile Telecommunication System, i.e.IMT-2000 or Universal Mobile Telecommunication System (UMTS). It is acommon sense that the CDMA techniques will be deployed on the thirdgeneration communication system. Among the CDMA techniques the WCDMA isan important air interface scheme and is greatly concerned.

In the present WCDMA protocol, each cell has its own clock, but there isno unique reference time among cells. Therefore, relative timingrelationship of different channels in a cell is definite, but timingrelationship among cells is unknown. This means that every channelwithin a NodeB is synchronous, but channels among different NodeBs areasynchronous. In other words, User Equipment (UE) knows timing of thesignals in its own cell but does not know any timing information aboutother cells. Problems arise in this situation.

1) Timing drift among different NodeBs for a long time may causeoverlaps of downlink synchronization channels so that the downlinksynchronization channels are unable to be differentiated, and the UE isimpossible to find a adjacent cell so that a handover can not be made.Since the UE has no timing information of other cells, it is necessaryto search the adjacent cells in all phases; the result is that resourceis wasted, design is more difficult and at the same time powerconsumption of the UE is great so that its await time is shorten.

2) During soft handover, the UE must measure the arriving timedifferences of the signals from adjacent cells in order to ensure thatdelay-time of the downlink traffic channels among cells in active setsare minimum to take advantage for combination. Nevertheless,asynchronies among cells make the UE more complicated; the arriving timeof multipath signals is difficult to be measured accurately and isvaried as propagation environment. After the UE has reported upward thearriving time differences of signals from different cells, the targetNodeB making soft handover must adjust the transmitting moment of thedownlink traffic channel to obtain a minimum delay-time difference forthe traffic channels. For supporting the soft handover and downlinktransmitting moment adjustment, a complicated signaling procedure mustbe introduced; this not only increases the communication between RadioNetwork Controller (RNC) and NodeB, but also decreases the handoverspeed, success and reliability, which affect the system capacity andservice quality.

SUMMARY OF THE INVENTION

Based on what has mentioned above, objective of the invention is toprovide a method that implements synchronization of cells in a WCDMAsystem. With this method, synchronization is kept among every NodeB ofthe system; so difficulty of UE design is decreased, await time of theUE is increased, signaling procedure of handover is simplified and thesuccess and reliability of handover is increased; all of these willimprove system capacity and service quality level.

For above objective, the invention implements with the followingtechnical scheme:

A method for synchronization in a WCDMA system, comprises, introducing aglobe synchronization time as a reference time for signal transmittingmoment of all base stations in the WCDMA system. Wherein the globesynchronization time is an even second moment of Globe PositioningSystem (GPS).

The method further comprises, transmitting a common parameters messageto a User Equipment (UE) by a current cell NodeB; computing arrivalmoment of a common pilot channel (CPICH) frame of an adjacent cell withcommon parameters in the common parameters message by the UE, andsearching the adjacent cell at the time that near the said arrivalmoment. Wherein the adjacent cell is every cell that affects signals ofthe current cell and the common parameters message at least includes acell time-offset of the current cell, a cell time-offset and a CPICHprimary scrambling code of the adjacent cell.

Wherein the cell time-offset equals a minimum delay-time betweenstarting moment of the downlink CPICH frame of the said cell and thereference time, which is an integer multiple of 256 chips and isallocated by a Base Station Controller (BSC). The integer is from 0 to149.

The method further comprises, for more than one adjacent cell,transmitting a common parameters message that includes the celltime-offsets and the CPICH primary scrambling code respectivelycorresponding to each adjacent cell to the UE by the current cell NodeB.

The method, further comprises, for more than one adjacent cells,transmitting more than one common parameters message to the UE, whereineach message corresponds to the cell time-offset and the CPICH primaryscrambling code of one adjacent cell respectively, and number of thetransmitted message equals to number of the adjacent cells.

The method further comprises, when establishing a Dedicated PhysicalChannel (DPCH), transmitting the common parameters message to the UE bythe current cell NobeB, computing a DPCH frame arrival moment based onparameters in the said common parameters message and searching the DPCHat the time that near the computed arrival moment by the said UE.Wherein the said common parameters message includes a DPCH time-offsetof the said UE.

Wherein the DPCH time-offset equals the minimum delay-time betweenstarting moment of the DPCH frame and the reference time, which is aninteger multiple of 256 chips and is allocated to the base station bythe BSC. The integer is from 0 to 149.

The method further comprises, for an adjacent cell with differentfrequency, adding a different-frequency common parameters message in aDPCH of the current cell and transmitting to the UE by the current cellNodeB.

Wherein the different-frequency common parameters message at leastincludes the cell time-offset, the CPICH primary scrambling code and afrequency of the adjacent cell with different frequency.

The method further comprises, when the UE makes soft handover, settingstarting moment of the DPCH frame in a target cell same as startingmoment of the DPCH frame in a source cell.

It can be seen from the mention above that the key of the invention isthat a synchronization mechanism is introduced in the WCDMA system, i.e.a standard reference time that ensures synchronization of transmittingmoment of all NodeBs is introduced to the WCDMA system.

The advantages and features of the invention are as follow.

1) In the WCDMA system, every adjacent cell NodeB is synchronized to theGPS time signal; time-offset can be defined through the networkplanning, i.e. through taking account of various facts relating to cellsin the wireless cellular communication system, so transmitting overlapof the Synchronization Channel (SCH) and CPICH is avoided.

2) Through the transmitted common parameter message, the UE knows thetiming relationship and scrambling code of the current cell, theadjacent cell or the adjacent cell with different frequency; so duringsearching the adjacent cell, the UE needs only to search near a definedmoment and only uses the scrambling code to search directly on theCPICH; in this way, the searching procedure is simplified, the searchingtime is shorter, and the UE design complexity is reduced, powerconsumption of the UE is reduced so that its await time is increased.The service quality is guaranteed.

3) During soft handover, when defining the transmitting moment of thedownlink DPCH of a target cell, the DPCH time-offset of the user in thesource cell may be used directly, and there is no necessary to measurethe downlink propagation delay-time of signals from different basestations; during hard handover, the rough relationship of thetransmitting moment and scrambling code of the source cell and thetarget cell is known, so the handover procedure can be speeded up sothat handover success and reliability of handover is increased and theservice quality is improved.

4) Through measurement the UE knows the arriving time difference ofsignals from the two cells, and the difference of these two arrivingtime differences is the propagation delay-time difference from theirNodeBs to the UE since CPICH frame transmitting moments of the two cellsare known, i.e. the transmitting time difference between them is known;so with this difference, distances difference from NodeBs of the twocells to the UE can be computed. Suppose there are three cell NodeBs,the UE can know the distances between any two NodeBs with the same way,so accurate position of the UE is obtained. It is seen thatsynchronization between base stations makes advantage for mobile stationpositioning.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the timing diagram of each channel for different cells ofthe invention in the WCDMA system.

EMBODIMENTS OF THE INVENTION

The invention will be described in more detail with reference to thedrawing and embodiment.

In this invention, a time signal of the Globe Position System (GPS) orsimilar time signal of globe synchronization are introduced as thestandard reference time in the WCDMA system in order to synchronize timesignals of all base stations. In this embodiment, the GPS even secondmoment is taken as the standard reference time.

Timing relationship of the cell in the WCDMA system is shown in FIG. 1after the synchronization has been introduced; all the time measureposition is defined at an antenna connector of the NodeB. Timingrelationship of channels in a same cell is unchanged, which isconsistent with the WCDMA protocol; transmitting moment of every celltakes the introduced reference time, and relating to the standardreference time, starting moment for CPICH frame of each cell is offset.It is defined that a cell time-offset, which is an integer multiple of256 chips, is the minimum delay-time between the downlink CPICH framestarting moment and the GPS even second moment; the cell time-offset isobtained from the network planning and is allocated to the NodeB by theBSC. In FIG. 1, for example, Cell 1 has a time-offset of 256*n chips,where n is an integer on [0 . . . 149], and this means that the startingmoment of CPICH frame in Cell 1 has a minimum delay-time of 256*n chipsfrom the GPS even second moment; Cell 2 has a time-offset of 256*mchips, where m is an integer on [0 . . . 149], and this means that thestarting moment of CPICH frame in Cell 2 has a minimum delay-time of256*m chips from the GPS even second moment. In order to avoid SCHs ofadjacent NodeBs overlapping, it is better to select a larger differenceof time-offset among adjacent cells, i.e. m is not equal to n and has alarger difference; it is suggested that time-offset difference amongadjacent cells is greater than 2*256 chips. What said network planningis to plan the time-offset of a wireless cellular communication systemaccording to certain rules based on systemization of various parameters,such as geographical location, size and shape of the cells and overlaparea etc.

Similarly, a DPCH time-offset, which is an integer multiple of 256 chipsand set by upper layer, is the minimum delay-time of the downlink DPCHframe starting moment that is relative to the GPS even second moment. Asshown in FIG. 1, in Cell 1 the downlink DPCH time-offset is 256*i chips,where i is an integer on [0 . . . 149]; this means that the minimumdelay-time of the DPCH frame starting moment is 256*i chips. The celltime-offset and DPCH time-offset must be kept within a tolerance toavoid time drift for a long period. In general, the tolerance is lessthan 3 ms. Timing relationships of other channels are in accordance withthe WCDMA protocol.

After the cell time-offsets have been defined, the common parametersmessage is transmitted down to the UE through the downlink commonchannel in each cell by the NodeBs; the parameters include mainly thecell time-offset of the current cell, the cell time-offsets and theCPICH primary scrambling codes of adjacent cells. The common parametersmessage is transmitted circularly; and the circular interval will bedefined. The said adjacent cells are the cells that affect the currentcell signal and may not be geographically adjacent cells. The commonparameters message of every adjacent cell must be transmitted down inthe current cell; the NodeB may transmit the cell time-offsets and CPICHprimary scrambling codes of all adjacent cells with one message, or inthe CPICH with several messages respectively.

After having received the common parameters message, the UE searches theadjacent cell with the parameters and the procedure can be explainedwith reference to FIG. 1. When Cell 1 is the current cell of the UE, theUE obtains Cell 1 time-offset n, Cell 2 time-offset m and CPICH primaryscrambling code of Cell 2, from the Cell 1 downlink common parametermessage. Without taking into account propagation delay-time from theNodeB to the UE, the UE will deduce the reference time of the GPS evensecond moment T according to the arrival moment t1 of the Cell 1 signaland Cell 1 time-offset of 256*n chips; the reference time T plus Cell 2time-offset of 256*m chips are the arrival moment t2 of the CPICH frameof the adjacent cell, Cell 2. When taking into account propagationdelay-time, t2 is only an approximate arrival moment of the CPICH framein the Cell 2, since propagation delay-time for different UE may bedifferent; in this case, when the UE searches the adjacent cell, it isneeded to search some phases near moment t2, but not all the phases.Furthermore, since the UE knows the CPICH primary scrambling code of theadjacent cell by the common parameters message, the searching may betaken directly on the CPICH. In this way, the searching adjacent cellprocedure is greatly simplified.

If there are adjacent cells with different frequency, a differentfrequency common parameter message is transmitted down to the UE throughthe downlink common channel in the current cell, including thefrequency, the all time-offsets and the CPICH primary scrambling codesof the adjacent cells with different frequency. Then, when the UEsearches the adjacent cell with different frequency, it is unnecessaryto search all phases and to take the complicated initial accessprocedure, but only makes the primary scrambling code matching directlyon the CPICH near a known phases. The searching procedure of theadjacent cell with different frequency is same as the searchingprocedure of the adjacent cell with same frequency except that thesearching is at a time interval of compression mode and at a differentfrequency. Therefore, the hard handover speed is greatly increased andthe service quality is improved.

When establishing the DPCH of the UE, the NodeB of the current cellallocates its DPCH time-offset i and sends messages to the UE; thenaccording to the parameters involved in the messages, the UE computes anarrival moment of the DPCH frame and searches the DPCH near the arrivalmoment.

When a soft handover condition is satisfied, the RNC will designate thesoft handover. Since the starting moment of the DPCH frame for the UE atthe source cell is known, in general it is enough to set the same DPCHframe starting moment at the target cell. As shown in FIG. 1, supposethe UE in Cell 1 has DPCH time-offset of 256*i chips, and the UE willmake soft handover to Cell 2; when establishing the downlink DPCH inCell 2, it is enough to allocate the DPCH time-offset with same 256*ichips. In this case, the downlink DPCHs of the two cells, Cell 1 andCell 2, are transmitted from their NodeBs synchronously, and arrive atthe UE spending its own propagation time respectively; the arriving timedifference at the UE equals the propagation delay-time difference fromthe two NodeBs to the UE. Since usually the soft handover is happened inadjacent cells and the propagation delay-time difference of theseadjacent cells are usually within the combination allowance, such aswithin 128 chips, it is unnecessary to make any adjustment, i.e. withoutany adjustment the downlink of different NodeBs will be combinedcorrectly in soft handover procedure; even the propagation delay-timedifferences are great, they can be compensated by allocating the DPCHtime-offset of the target cell through the network planning. Therefore,it is unnecessary to make accurate measurement arriving time differencesof the signals from cells for synchronization; this simplifies thehandover procedure, reduces a lot of higher layer signaling, reducessystem load and increases handover success and reliability.

In the hard handover, the similar synchronization mechanism mentionedabove can also be used to reduce the handover complexity and increasethe hard handover success and reliability.

1. A method for synchronization in a Wideband Code Division MultipleAccess (WCDMA) system, comprises, introducing a globe synchronizationtime as a reference time for signal transmitting moment of all basestations in the WCDMA system.
 2. The method according to claim 1,setting an even second moment of Globe Positioning System (GPS) as thereference time for signal transmitting moment of all base stations inthe WCDMA system.
 3. The method according to claims 1, furthercomprises, transmitting a common parameters message to a User Equipment(UE) by a current cell NodeB; computing arrival moment of a common pilotchannel (CPICH) frame of an adjacent cell according to common parametersin the common parameters message by the UE, and searching the adjacentcell at the time that near the said arrival moment.
 4. The methodaccording to claim 3, wherein the adjacent cell is every cell thataffects signals of the current cell.
 5. The method according to claim 3,wherein the common parameters message at least includes a celltime-offset of the current cell, a cell time-offset and a CPICH primaryscrambling code of the adjacent cell.
 6. The method according to claim5, wherein the cell time-offset equals a minimum delay-time between thestarting moment of the downlink CPICH frame of the said cell and thereference time, which is an integer multiple of 256 chips and isallocated by a Base Station Controller (BSC).
 7. The method according toclaim 6, the integer is from 0 to
 149. 8. The method according to claim5, further comprises, if there are more than one adjacent cell,transmitting a common parameters message that includes the celltime-offsets and the CPICH primary scrambling code respectivelycorresponding to each adjacent cell to the UE by the current cell NodeB.9. The method according to claim 5, further comprises, if there are morethan one adjacent cell, transmitting more than one common parametersmessage to the UE, wherein each message corresponds to the celltime-offset and the CPICH scrambling code of one adjacent cellrespectively, and the number of the transmitted message equals to thenumber of the adjacent cells.
 10. The method according to claim 1,further comprises, when establishing a Dedicated Physical Channel(DPCH), transmitting the common parameters message to the UE by thecurrent cell NobeB, computing a DPCH frame arrival moment based onparameters in the said common parameters message and searching the DPCHnear the computed arrival moment by the said UE.
 11. The methodaccording to claim 10, wherein the said common parameters messageincludes a DPCH time-offset of the said UE.
 12. The method according toclaim 11, wherein the DPCH time-offset equals the minimum delay-timebetween starting moment of the DPCH frame and the reference time, whichis an integer multiple of 256 chips and is allocated to the base stationby the BSC.
 13. The method according to claim 12, the integer is from 0to
 149. 14. The method according to claim 3, further comprises, if anadjacent cell with different frequency, adding a different-frequencycommon parameters message in a DPCH of the current cell and transmittingto the UE by the current cell NodeB.
 15. The method according to claim14, wherein the different-frequency common parameters message at leastincludes the cell time-offset, the CPICH primary scrambling code and afrequency of the adjacent cell with different frequency.
 16. The methodaccording to claim 1, further comprises, when the UE makes softhandover, setting starting moment of the DPCH frame in a target cellsame as starting moment of the DPCH frame in a source cell.